A study by an INRA team, published in the journal, Physiology & Behavior, also showed that rainbow trout can discriminate between diets containing different level of omega-3 fatty acids (ω−3 LCPUFA).
“The levels of ω−3 LCPUFA drove dietary choices in the fish,” found the researchers.
In the current context, where aquaculture reliance on marine products must be reduced, several studies have observed that the use of plant based-diets creates challenges such as a reduction in feed-intake, growth performance or a combination of lower feed intake and reduced feed efficiency, said the researchers. This documented decrease in growth performance linked to plant based diets is believed to be mainly related to the absence of marine products, fish meal and particularly fish oil but the physiological explanation of this remains remain poorly documented, said the INRA team.
To date, they noted, studies that have investigated the impact of the substitution of fishmeal and fish oil with alternative dietary products such as plants, insects, yeast and algae have focused their scope on fish performance, metabolism and functional genomics.
Increased feed conversion efficiency and reduced nutrient losses are major objectives in intensive aquaculture and, as such, it is, therefore, important to understand the feeding behavior of farmed fish, they stressed.
"It seems clear that understanding the basic physiological mechanisms regulating feed intake of farmed fish will allow industry to enhance feed conversion efficiency and reduce nutrient losses."
The team's objective was an investigation into the feed preference of rainbow trout for three different dietary levels of ω−3 LCPUFA (DHA and EPA): low 0%, medium 5% and high 20% total fatty acid content in the diet.
Their hypothesis was that the absence of ω−3 LCPUFA in a plant-based diet would lead to a decrease of food intake of farmed fish due to the lack of palatability of such a diet.
The researchers outlined how the feed preference of the fish for each diet was measured during two periods of three weeks by means of self-feeders and their preference was tested by offering the choice between two of the three diets containing low, medium or high dietary levels of ω−3 LCPUFA.
“The first test period was followed by the second period when the diets were exchanged between feeders in order to observe whether the fish had learnt their preferences. Finally, we analyzed fish growth performance, feed intake variables (total consumed and uneaten food) and feed preferences (daily and cumulative in absolute and relative terms during the test and exchange periods).”
Diets were manufactured at the INRA experimental facilities in Landes, France, using a twinscrew extruder, said the researchers. Pellets, 3 mm in diameter and 3 mm in length, were produced.
Composition and proximate analysis of the three diets:
All diets were formulated based on the same feed ingredient composition, differing only in their oil derivation. A commercial diet, of marine and vegetable derivation, served as a reference diet, they said.
“In order to avoid exceeding anti-nutrient threshold levels, we used a blend of wheat gluten, extruded peas and whole wheat, corn gluten meal, rapeseed meal and white lupin as protein sources. Synthetic L-lysine, L-methionine, dicalciumphosphate and soy-lecithin were added to all diets to correct the deficiency in essential amino acids, phosphorous and phospholipids.”
A mineral and a vitamin premix was added to each diet. Diets were isoenergetic and were formulated to cover the nutrient requirements of rainbow trout, added the researchers.
“In order to maintain a constant ratio between groups of fatty acids (saturated, monounsaturated, ω−3 PUFA, ω−6 PUFA and ω−9 PUFA), the three experimental diets differed by the level of DHA and EPA to the benefit of alpha-linolenic acid (ALA), the ω−3 PUFA precursor of these two fatty acids. To do this, the dietary content of EPA and DHA was increased by adding Omegavie marine oils DHA and EPA. The three experimental diets contained 23.66% crude lipids with 0.25% (devoid of DHA) of ω−3 DHA/EPA (% of total fatty acids) for low, 4.92% for medium and 19.59% for high ω−3 diets.”
The results showed that rainbow trout could discriminate between the diets containing different level of ω-3 LCPUFA.
Overall, the team saw that the fish preferred a diet high in ω-3 LCPUFA: 59.5% preference for high ω-3 diet in high v. low ω-3 diets, and 75.6% preference for high ω-3 diet in medium v. high ω-3 diets respectively. This preference was repeated after 21 days and for a further 21 days when the feeds were exchanged between the two self- feeders in each tank: 63.3% preference for high ω-3 diet in high v. low ω-3 diets, and 69,5% preference for high ω-3 diet in medium v. high ω-3 diets respectively, said the INRA team.
The tests also indicated a difference in the extent of food waste of each of the three diets, demonstrated by uneaten pellets, they added. During two periods of testing, high ω-3 diet was the most appreciated, the least wasted and the most eaten, whereas the most uneaten feed remained the least appreciated diet in three choices diets (low ω-3 diet in low v. medium ω-3 diets, medium in medium v. high ω-3 diets and low in low v. high ω-3), they found.
Summarizing, the researchers said their data highlights the influence of ω-3 LCPUFA in the feeding behavior of juvenile rainbow trout.
“Further studies will be necessary to correlate the feed preference for diets higher in ω−3 LCPUFA and the implications on the central control of food intake particularly the fatty acid sensing pathways and brain reward system or hedonic feeding in farmed fish.”
Source: Physiology & Behavior
Title: Rainbow trout prefer diets rich in omega-3 long chain polyunsaturated fatty acids DHA and EPA
Authors: J. Roy et al